Circuit for protecting contacts against damage from arcing

ABSTRACT

This disclosure deals with a circuit for substantially reducing or eliminating switch contact damage due to arcing when opening or closing the switch contacts under heavy AC current load conditions. This is accomplished by providing a normally open shunt path across the contacts and momentarily closing the path at the instant of arcing. The shunt path includes contactless switch means which is normally open. Opening of the switch contacts effects turning on of the switch means and transfer of the load current thereto, thus quenching the arc. At the next subsequent zero crossing of the AC wave, the switch means is again opened. Similar operation occurs when the switch contacts close and an arc starts to develop due to contact bounce.

United States Patent [191 Schaefer et al.

[451 Feb. 25, 1975 CIRCUIT FOR PROTECTING CONTACTS AGAINST DAMAGE FROM ARCING [75] Inventors: Edward J. Schaefer; August L. Streater, both of Bluffton, Ind.

[73] Assignee: Franklin Electric Co., Inc., Bluffton,

Ind.

[22] Filed: Apr. 26, 1973 [21] App]. No.: 354,694

[52] US. Cl. 317/11 E, 307/136, 317/33 SC [51] Int. Cl. H02h 7/22 [58] Field of Search 317/11 E, 33 SC; 307/136 [56] References Cited UNITED STATES PATENTS 10/1969 Siwko et a1 317/11 E 1/1971 Casson 317/11 E Primary Examiner-James D. Trammell Attorney, Agent, or Firm-Hibben, Noyes & Bicknell [57] ABSTRACT This disclosure deals with a circuit for substantially reducing or eliminating switch contact damage due to arcing when opening or closing the switch contacts under heavy AC current load conditions. This is accomplished by providing a normally open shunt path across the contacts and momentarily closing the path I 32 Claims, 11 Drawing Figures PATENTED FEB 25 1925 SHEET 1 2 PATENIEB FEB 25 ms SHEET 2 [1F 2 CIRCUIT FOR PROTECTING CONTACTS AGAINST DAMAGE FROM ARCING In many electrical installations it is necessary to open or close an electric switch under heavy current load conditions. For example, in an AC electric motor including an overload switch, the switch is designed to open and disconnect the motor winding from the AC power supply to protect the motor during such conditions as arise from a locked rotor, and with a current load that may be over 100 amperes when the switch opens. Arcing occurs when the switch opens, and unless a large overload switch is used, the switch contacts may be severly damaged or destroyed after opening a few times under such conditions. In addition, arcing may also occur if the switch contacts are closed under heavy current load conditions because the contacts tend to bounce afer initial closure. When the contacts initially close, current flows through them, and if the contacts bounce open again immediately after initial closure, an arc develops which can destroy the contacts.

It would of course be possible to use a large heavyduty switch having an ample life at such current breaking loads, but quite often the necessary space is not available for such a switch. Further, even if space were not a problem, it would be advantageous to be able to extend the life, or increase the rating, of a switch of any given size.

In numerous other instances, contact-type switches are used to interrupt current flow in a circuit, and arcing can reduce the life of the switches.

It is therefore a general object of the present invention to provide circuit means for use with switch contacts, to eliminate or reduce damage due to arcing, thereby enabling the use of a smaller switch than would otherwise be required and/or prolonging the life of the switch.

The foregoing general object is attained by providing, in a circuit including switch contacts connected to carry alternating current and required to open under current load conditions, the improvement comprising contactless switch means adapted to be connected in parallel with said switch contacts, said switch means including bias means connected to respond to current flow through said contacts and to a potential across said contacts, said switch means being biased to conduction only when a potential exists across said contacts and current flows through said contacts, whereby said switch means is biased to conduction and load current is transferred thereto from the time of initiation of an arc until the next subsequent zero crossing of the alternating current.

The foregoing and other aspects of the invention will be apparent from the following detailed description taken in conjunction with the accompanying figures of the drawings, wherein:

FlG. 1 is a schematic electrical diagram of a circuit embodying the invention; and

FIGS. 2 through 11 are diagrams-similar to H6. 1 but showing alternate forms of the invention.

While some of the forms of the present invention are illustrated and described in connection with an overload switch for an electric motor, it should be understood that the invention has utility in a variety of other electrical installations where contacts are used to interrupt current flow in a circuit.

The circuit shown in FIG. 1 includes a load 10 which is connectable. to be energized by an AC power supply 9, and by a switch 11 including contacts 12. A low value resistor 16 is connected in series with the switch 11 for a purpose to be described hereinafter. Connected in parallel with the switch 11 is contactless switch means which preferably is of the solid state type. The switch means may be a TRIAC, a pair of back-toback parallel connected SCRs or thyratrons, and in the present instance, a TRIAC 17 is provided. One main or power terminal of the TRIAC 17 is connected to the juncture of the load 10 and the contacts 12, and the other main or power terminal is connected to the juncture of the contacts 12 and the resistor 16. The gate 18 of the TRIAC 17 is connected through a resistor 19 to the other side of the resistor 16. The resistor 19 is provided to limit the TRIAC gate current to within the gate dissipation rating of the TRIAC.

Considering the operation of the circuit illustrated in FIG. 1, assume that the switch 11 has been closed to connect the load 10 to the AC power supply 9, and that current is flowing through the load 10, the switch 11 and the resistor 16. The potential across the resistor 16 energizes the gate 18 of the TRlAC 17, but no current flows through the TRIAC 17 because the closed switch contacts 12 short the TRIAC 17. The TRIAC 17 requires a potential of at least approximately 1.5 volts before it will conduct, and the potential across the switch 12 is less than this value. Consequently, even though the gate 18 is energized, the TRlAC 17 normally does not conduct.

If the contacts 12 are opened, an arc starts to develop upon initialopening movement of the contacts. The are potential may be, for example, about 30 volts which of course is sufficient to turn on the TRIAC 17, and the current flow during initial arcing flows through the resistor 16 and continues energization of the gate 18. Consequently, the TRlAC 17 is turned on instantly as soon as the arc starts to develop, and the power current is transferred away from the contacts 12 to the TRlAC 17. Therefore, the arc is immediately quenched or extinguished, and the contacts 12 are not damaged. The power current continues to flow through the TRIAC 17 until the next subsequent zero crossing of the AC wave, at which time the TRIAC 17 is turned off because of the lack of a potential across it. In the next subsequent AC half cycle, the AC line potential appears across the TRIAC but it does not conduct because no current flows through the resistor 16 as is required to energize the gate 18. The gate current is in phase with the load current and both reduce to zero at the same time, this being true because the element 16 is a resistor. Consequently, the TRIAC l7 conducts for less than one half of an AC wave, and while the current flow through the TRlAC may be very high during this time, it is relatively short and an ordinary TRlAC can handle such current flow without the necessity of providing a sink to draw heat away from the TRlAC. The TRlAC 17 is a bistable contactless switch which includes bias means connected to respond both to the are potential across the switch 11 and to current flow through he switch.

If the switch 11 is later closed, current again flows therethrough, and if the contacts 12 bounce immediately after closing, an arc tends to develop during the bounce. However, the flow of current through the resistor 16 occurring as soon as the contacts 12 initially close, causes the TRlAC gate 18 to be energized, and

the initiation of an are at the time of the contact bounce results in a potential across the TRIAC 17 which turns it on as described previously. Consequently, the TRIAC 17 again conducts and draws the current away from the switch 11, permitting the switch 11 to reclose under relatively light current flow conditions. Of course, as soon as the contacts 12 reclose, the TRIAC 17 is again shorted and is turned off.

The foregoing sequence of contact closure and bounce, arcing, turning on of the TRIAC, reclosure and turning off of the TRIAC, occurs as described when all of the events occur within the same half wave. If the bounce and arc occur in a first half wave and the reclosure occurs in the next half wave, the TRIAC will be turned off at the zero crossing and the arc may reestablish at the beginning of the next half wave. In this event, the TRIAC would again be turned on during the next half wave and would remain on until the next zero crossing or until the contacts reclose, whichever occurs first.

The size of the resistor 16 should of course be made no larger than necessary, to prevent waste of power and generation of heat. Its size is determined by the amount of gate current required to trigger the TRIAC l7, and depending upon the TRIAC requirement, it may be made very small. Further, with a proper choice of component values, the current limiting resistor 19 may be eliminated in the FIG. I circuit and in the other circuits disclosed herein.

In some instances, as where the switch 11 is a current overload switch, a small voltage may develop across the TRIAC main terminals in the absence of an arc when current values become large, and this condition may exist for a number of AC cycles before the switch contacts open. This small voltage may exceed the voltage required to turn on the TRIAC and cause it to conduct for a number of cycles, resulting in overheating of the TRIAC if adequate heat sinking is not provided. The foregoing may be prevented by using one of the forms of the invention shown in FIGS. 2 and 3.

The form of the invention shown in FIG. 2 includes a current overload switch 21, having contacts 22 and a heater resistor 23, connected in series with an AC power supply 24, an on-off switch 26, a motor winding 27, and a shunt resistor 28. The motor also includes a rotor 29. A TRIAC 31 and a zener diode 32 are connected in series and are connected across the overload switch 21. The gate 33 of the TRIAC is connected through a current limiting resistor 34 to the resistor 28. In one conventional type of overload switch, the contacts 22 are part of a bimetal element which bends in response to the heat generated by the heater 24. The overload switch 21 is designed such that the bimetal element is normally closed but deforms and opens the contacts 22 when sufficient current flows through the heater 23. The switch 21 is designed to open upon the occurrence of overload currents, as when the rotor 29 of the motor is locked. When the switch cools, the contacts 22 automatically close again.

In overload conditions but before the switch contacts 22 open, if a voltage develops across the switch 21, the zener diode 32 prevents any current flow through the TRIAC 31 unless the zener voltage plus the TRIAC 31 turn-on voltage is reached. The voltage across the switch 21 would normally be less than 2 volts, and a zener is chosen having a higher voltage, thus preventing current flow through the TRIAC until the switch contacts 22 open and an arc starts to appear. The remainder of the operation is the same as that described with regard to FIG. I.

In the form of the invention illustrated in FIG. 3, a pair of diodes 36 and 37 are connected in back-to-back parallel relation, and the parallel doides 36 and 37 are connected in series with a TRIAC 38. The diodes have a forward bias voltage of about 1.5 to 2 volts, and the diodes 36 and 37 prevent current flow through the TRIAC 38 until the potential across an overload switch 39 exceeds the diode and the TRIAC minimum voltages. The remainder of the circuit, and the operation, are similar to that of FIG. 2.

In the form of the invention illustrated in FIG. 4, a TRIAC 41 is connected in parallel with the contacts 42 of an overload switch 43 which also includes a heater resistor 44. The gate 46 of the TRIAC is connected through a current limiting resistor 47 to the side of the heater resistor 44 which is opposite from the contacts 43. The circuit shown in FIG. 4 may be used when the connection between the contacts 42 and the resistor 44 is accessible. It will be apparent that the circuit of FIG. 4 will operate similarly to the circuit of FIG. 1, and that the heater resistor 44 both heats the contacts 42 and also serves the function of the resitor 16 of FIG. 1. In the FIG. 4 circuit, the TRIAC turn-on voltage probably will not be reached before the contacts 42 open, because the TRIAC is not connected across the heater resistor 44. Therefore, it is less likely that a diode arrangement as shown in FIGS. 2 and 3 would be required, although a diode arrangement may be provided in the FIG. 4 circuit or any of the circuits disclosed herein.

The circuit illustrated in FIG. 5 includes an over-load switch 51 connected in series with a load 52 and an AC power supply 53, the switch 51 including contacts 54 and a heater resistor 55. Also connected in series with the overload switch 51 is a shunt resistor 57. A TRIAC 58 is connected across the switch 51, and the gate 59 of the TRIAC 58 is connected through a current limiting resistor 61 to the resistor 57.

It will be apparent that the construction and operation of the FIG. 5 circuit is similar to that of the FIG. 4 circuit, except that the TRIAC gate current is provided by a shunt resistor separate from the overload switch. In the arrangement shown in FIG. 5, the value of the potential across the main or power terminals of the TRIAC 58 prior to the opening of the contacts 54 is not sufficient to turn the TRIAC 58 on. If the TRIAC 58 were to be turned on with the contacts 54 closed, a

heat sink may be provided to carry heat from the TRIAC 58 in the event it is on long enough for heating to become a problem.

The circuit shown in FIG. 6 includes switch contacts 63 connected in series with a load 64 and an AC power supply 66. A small current transformer 67, including a primary winding 68 and a secondary winding 69 is connected to supply gate energizing current to the TRIAC 71. The primary winding 68 is connected in series with the contacts 63 and the secondary winding 69 is connected through a current limiting resistor 72 to the gate 73 of the TRIAC 71. The amount of gate energizing power required is of course very low, measured in milliwatts. The transformer 67 may therefore be in the order of a 300 to 1 step down current transformer, for example.

Assuming that the switch contacts 63 are closed, current flows through the load 64, the contacts 63 and the primary winding 68. The alternating current flowing through the primary winding 68 induces current flow in the secondary winding 69-,and supplies gate energizing current to the TRIAC 71. The main terminals of the TRIAC '71 are connected across the switch contacts 63 and the primary of winding 68, instead of across the switch contacts only as in the circuit shown in FIG. 1. Since the transformer 67 primary voltage drop may be made very small due to its very small rating (of the order of milliwatts), this drop can easily be limited to a value which will not cause the TRIAC 71 to conduct when the switch 63 is closed. Moreover, the resultant gate current flowing in the secondary winding 69 may be out of phase with and lead the main current by an appreciable phase angle. However, since the main terminals of the TRIAC are connected both across the primary of transformer 67 and across the contacts 63, when the contacts 63 open and the load current is transferred to the TRIAC, the primary winding of the transformer 67, and therefore also the gate 73, will cease to carry current. With no gate energizing current, the flow of current through the TRIAC cannot be reestablished after the power current reaches zero at the completion of the half cycle of conduction. Thus, with the connection of the main terminals of the TRIAC as shown in FIG. 6 rather than as shown in FIG. 5, the phase angle of the gate current relative to the load current phase angle is immaterial.

The circuit illustrated in FIG. 7 is generally similar to that illustrated in FIG. 6 but includes a voltage transformer rather than a current transformer. The FIG. 7 circuit includes switch contacts 76, a load 77 and an AC power supply 78, The components 76, 77 and 78 being connected in series with a small shunt resistor 79. A volage transformer 81 is provided, having its primary winding 82 connected across the resistor 79 and its secondary winding 83 connected to the gate 84 of a TRIAC 86. The value of the resistor 79 may be much smaller than the shunt resistor of the circuits of the character shown in FIG. 1, for example, and the transformer 81 may be, for example, a one-tostep up transformer. The power terminals of the TRIAC 86 are connected across the contacts 76 and the resistor 79.

With the contacts 76 closed, current flows through the resistor 79 and the potential across the resistor 79 induces gate energizing current in the secondary winding 83. The remainder of the operation is similar to that of the circuit illustrated in FIG. 6.

It might be mentioned that the circuits including transformers are advantageous in that the amount of power losses and heating, inherent in purely resistive shunts, are greatly reduced. Further, because of the re duction in power rating to only slightly greater than the milliwatts requirement of the TRIAC gate circuit, the transformer design can be made very simple, will require much less space, and is inexpensive as compared to resistive shunts of the required rating.

The circuit illustrated in FIG. 8 includes switch contacts 88 connected in series with a load 89, an AC power supply 91 and a small shunt resistor 92. A TRIAC 93 has its power terminals connected across both the contacts 88 and the shunt resistor 92, and the gate 94 of the TRIAC 93 is connected through a current limiting resistor 95 to the juncture of the contacts 88 with the resistor 92.

When the contacts 88 are closed, current flows through the contacts 88 and the resistor 92, and the potential across the resistor 92 supplies gate energizing current to the TRIAC 93. The TRIAC 93 is of course normally turned off when the contacts 88 are closed, but a heat sink may be provided to protect the TRIAC 93 in the event it turns on prior to the opening of the switch contacts 88.

The TRIAC is turned on by an are potential and by the gate energizing current. After the arc is extinguished, the TRIAC is turned off at the next zero crossing.

The circuit shown in FIG. 9 includes a current transformer similar to that shown in FIG. 6, but the TRIAC main terminals are connected across the switch contacts only, rather than across both the contacts and the transformer as in FIG. 6. The FIG. 9 circuit includes switch contacts 98 connected in series with a load 99, an AC power supply 101, and the primary winding 102 of a current transformer 103. The secondary winding 104 of the transformer 103 is connected through a current limiting resistor 106 to the gate 107 of a TRIAC 108. The power terminals of the TRIAC 108 are connected directly across the contacts 98.

The operation of the circuit shown in FIG. 9 will be the same as that of the circuit illustrated in FIG. 6, with the exception that the potential across the TRIAC main terminals will be lower than the potential across the main terminals of the TRIAC 71 because the TRIAC 98 is connected across the switch contacts 98 only. Further, in the circuit shown in FIG. 6, after the contacts 63 open and while the TRIAC 71 is still conducting, gate energizing current is no longer provided by the transformer 67. This is in contrast to the circuit shown in FIG. 9 where the TRIAC current flows through the transformer primary winding 102 and thereby maintains gate energizing current while the TRIAC 108 is conducting. After the TRIAC has been turned on, at the next zero crossing of the AC wave, the gate energizing current should also be substantially zero to turn the TRIAC off and maintain it off. In this circuit the transformer 103 must be designed to maintain its secondary current, which supplies the gate circuit, substantially in phase with the load current. This is in contrast to the circuit of FIG. 6 where the gate current phase angle is independent of the load current phase angle, as previously described in connection with FIG. 6.

The circuit shown in FIG. 10 includes a voltage transformer similar to that shown in FIG. 7. The FIG. 10 circuit includes switch contacts 111 connected in series with a load 112, a power supply 113, and a very small or low value resistor 114. The primary winding of a transformer 116 is connected across the resistor 114 and the secondary winding of the transformer 116 is connected through a resistor 117 to the gate 118 of a TRIAC 119. The main terminals of the TRIAC 119 are connected directly across the switch contacts 111.

The operation of the FIG. 10 circuit is similar to that of the circuit shown in FIG. 9 but differs in that the gate energizing current is provided by the potential across the resistor 114. In FIG. 10, and also in FIG. 7, the winding ratio of the transformer is determined by the value of the shunt resistor connected in series with the switch contacts, the amount of gate energizing current required and by the amount of current expected to flow through the resistor during arcing conditions.

However, the transformer 116 must be designed to maintain the gate current substantially in phase with the load current, as is also true of the transformer 103 in FIG. 9.

FIG. 11 shows a circuit including switch contacts 121 which are connected in series with a load 122, an AC power supply 123 and a reactive impedance which in the present instance is a winding 124. A TRIAC 126 has its main terminals connected across both the contacts 121 and the reactive impedance 124 which, in order to maintain losses and heating to a minimum should be highly reactive. The gate 127 of the TRIAC 126 is connected through a resistor 18 to the juncture of the contacts 121 and the winding 124. The reactance of the winding 124 would cause the TRIAC 126 gate current to be greatly out of phase with the load current, but, for the same reasons explained in connection with the circuit shown in FIG. 6, since there will be no gate current once the load current has been transferred to the circuit path including the TRIAC 126, there will be no re-establishment of the TRIAC current after it reaches the first zero crossing following the transfer of the load current. If, in order to obtain sufficient gate current, the voltage across the winding 124 became high enough to cause the TRIAC 126 to conduct while the contacts 121 are closed, a diode arrangement as shown in FIGS. 2 or 3 could be connected in series with the TRIAC 126.

It will be apparent from the foregoing that novel and useful circuits have been provided for preventing or substantially reducing damage to switch contacts which are required to open and close under heavy current load conditions. By using a circuit of the character described, a relatively small switch may be used which is able to accommodate overload currents but which may not be able to handle the opening and closing conditions encountered with conventional circuits. In installations where the amount of space is a critical factor, this is an important consideration. For example, an overload switch mounted within an electric motor must be quite small because of the limited amount of space available. By using the present invention, a small switch may be used and its life may be greatly extended compared to conventional constructions, because of the reduction or substantial elimination of contact damage due to arcing. The heat sink requirements of the TRIAC are negligible because of its short conducting time. In all of the forms disclosed herein, the contacttype switch is protected by a contactless switch which is normally open but which is closed upon the occurrence of an arc voltage and an arc current. Further, a variety of circuits or devices have been disclosed for supplying gate energizing current to a TRIAC. The gate energizing device is required to supply gate energizing current at the instant of arcing, and as mentioned, a variety of devices, such as a surge transformer connected to supply gate current only at the instant of contact opening, may be used for this purpose. Broadly, the circuits include contactless switch means connected to form a bypass current path around the contacts to be protected, the switch means including a gate or control terminal for closing it. In the circuits shown in FIGS 1 to 5, 9 and 10, it is important that the TRIAC gate current be in phase, or substantially in phase, with the load current because the TRIAC gate current continues after the contacts open and the arc is extinguished, making it possible for the TRIAC to re-establish conduction in subsequent cycles. In the remaining circuits disclosed herein, the phase angle of the gate current is unimportant but care should be taken to prevent the TRIAC from conducting when the contacts are closed.

We claim:

1. In a circuit including an alternating current supply, a load, and switch contacts connected to carry load current when closed and required to open under heavy current load conditions, the improvement comprising contactless switch means adapted to be connected to form a current path which bypasses said contacts, an electrical device adapted to be connected in series with said load and said contacts, said switch means including bias means connected to respond to current flow through said device and to a potential across said contacts, said switch means being biased to conduction only when a potential exists across said contacts and current flows through said device, whereby said switch means is biased to conduction and load current is transferred to said path from the time of initiation of an arc until the next subsequent zero crossing of the alternating current.

2. Apparatus as in claim 1, wherein said switch contacts and said device form a switch unit, and said device is a heater resistor.

3. Apparatus as in claim 1, wherein said switch means includes main terminals for carrying said load current and a control terminal for turning on said switch means, said main terminals being adapted to be connected to respond to a potential due to arcing, and said control terminal being'connected to said device, said device providing energizing current to said control terminal.

4. Apparatus as in claim 3, wherein said device comprises a resistor.

5. Apparatus as in claim 3, wherein said device comprises a current transformer.

6. Apparatus as in claim 3, wherein said device comprises a potential transformer and a resistor.

7. Apparatus as in claim 3, wherein said device comprises an impedance and said main terminals are connected across both said contacts and said impedance.

8. Apparatus as in claim 3, wherein said power terminals are adapted to be connected directly across said switch contacts.

9. Apparatus as in claim 3, wherein said power terminals are adapted to be connected across said switch contacts and said device.

10. Apparatus as in claim 1, wherein said switch means is a solid state device.

11. Apparatus as in'claim 1, wherein said switch means comprises a TRIAC having two main terminals and a gate, said main terminals being adapted to be connected across said contacts and said gate being adapted to be connected to respond to current flow through said device.

12. Apparatus as in claim 1, and further including diode means connected in series with said switch means to prevent current flow through said switch means until said potential exceeds a minimum value.

13. Apparatus as in claim 12, wherein said diode means comprises a zener diode.

14. Apparatus as in claim 12, wherein said diode means comprises two back-to-back parallel connected diodes.

15. An AC power circuit comprising electrical contacts adapted to be connected to a load and an AC power supply, an electrical device connected in series with said load and said supply, contactless switch means connected to form a current path which bypasses said contacts and including bias means responsive to a potential across said contacts and to current flow through said device, whereby said switch means conducts only from the time of opening of said contacts and initiation of an arc and until the next subsequent zero crossing of the alternating current.

16. Apparatus as in claim 15, and further including diode means connected in series with said switch means.

17. Apparatus as in claim 16, wherein said device comprises resistance means connected in series with said contacts, said switch means comprising a TRIAC having its gate connected to respond to current flow through said resistance means.

18. Apparatus as in claim 17, wherein said contacts and said resistance means form an overload switch.

19. Apparatus as in claim 16, wherein said device comprises a resistor.

20. Apparatus as in claim 16, wherein said device comprises a current transformer.

21. Apparatus as in claim 16, wherein said device comprises a potential transformer and a resistor.

22. Apparatus as in claim 16, wherein said switch means is adapted to be connected directly across said switch contacts.

23. Apparatus as in claim 16, wherein said switch means is adapted to be connected across said switch contacts and said device.

24. An AC power circuit for an electric motor, comprising a motor winding, overload switch contacts and a device mounted within said motor, said winding and said contacts and said device being connected in series and being adapted to be connected to an AC power supply, switch means mounted within said motor and connected to form a bypass current path around said contacts, said switch means including bias means responsive to a potential across said contacts and to current flow through said winding and said device, whereby said switch means is biased to conduction by a potential due to the initiation of an arc and continues to conduct until the next zero crossing of the AC wave.

25. Apparatus as in claim 24, wherein said switch means comprises a TRIAC, and further including diode means connected in series with said TRIAC.

26. Apparatus as in claim 24, wherein said device comprises a heater for heating said overload switch.

27. A circuit for preventing damage to switch contacts when opening under heavy current load conditions, comprising an electrical component adapted to be connected in series with said contacts and a load, voltage and current responsive switch means adapted to be connected to form a bypass current path around said contacts, said switch means including main terminals adapted to be connected across said contacts and a control terminal connected across said component, said switch means being biased to conduction only when a potential exists across said contacts due to arcing and when current flows through said component, whereby said current load is transferred from said contacts to said switch means from the initiation of an arc and until the next subsequent zero crossing of the alternating current.

28. Apparatus as in claim 6, wherein said main terminals are connected across both said contacts and said current transformer.

29. Apparatus as in claim 6, wherein said main terminals are connected across said contacts only.

30. In a power circuit including an AC power supply, a load connected to be energized by said supply, and switch contacts connected in series with said supply and said load for controlling the flow of current to said load, the improvement of means for preventing arcing across said contacts during opening under heavy load current conditions, comprising an electrical component connected in series with said contacts and said load, a triac including main terminals and a gate, said main terminals being connected in said power circuit to form a current path around said contacts, and said gate being connected to said component, whereby said triac is turned on when opening of said contacts produces a potential across said main terminals and current flow through said component provides gate triggering current.

31. In a power circuit as in claim 30, wherein said main terminals are connected directly across said contacts, said component is a resistor, and said gate triggering current is in phase with the voltage across said resistor.

32. In a power circuit as in claim 30, wherein said main terminals are connected across both said contacts and said component, whereby current flow through said triac bypasses both said contacts and said compo- 

1. In a circuit including an alternating current supply, a load, and switch contacts connected to carry load current when closed and required to open under heavy current load conditions, the improvement comprising contactless switch means adapted to be connected to form a current path which bypasses said contacts, an electrical device adapted to be connected in series with said load and said contacts, said switch means including bias means connected to respond to current flow through said device and to a potential across said contacts, said switch means being biased to conduction only when a potential exists across said contacts and current flows through said device, whereby said switch means is biased to conduction and load current is transferred to said path from the time of initiation of an arc until the next subsequent zero crossing of the alternating current.
 2. Apparatus as in claim 1, wherein said switch contacts and said device form a switch unit, and said device is a heater resistor.
 3. Apparatus as in claim 1, wherein said switch means includes main terminals for carrying said load current and a control terminal for turning on said switch means, said main terminals being adapted to be connected to respond to a potential due to arcing, and said control terminal being connected to said device, said device providing energizing current to said control terminal.
 4. Apparatus as in claim 3, wherein said device comprises a resistor.
 5. Apparatus as in claim 3, wherein said device comprises a current transformer.
 6. Apparatus as in claim 3, wherein said device comprises a potential transformer and a resistor.
 7. Apparatus as in claim 3, wherein said device comprises an impedance and said main terminals are connected across both said contacts and said impedance.
 8. Apparatus as in claim 3, wherein said power terminals are adapted to be connected directly across said switch contacts.
 9. Apparatus as in claim 3, wherein said power terminals are adapted to be connected across Said switch contacts and said device.
 10. Apparatus as in claim 1, wherein said switch means is a solid state device.
 11. Apparatus as in claim 1, wherein said switch means comprises a TRIAC having two main terminals and a gate, said main terminals being adapted to be connected across said contacts and said gate being adapted to be connected to respond to current flow through said device.
 12. Apparatus as in claim 1, and further including diode means connected in series with said switch means to prevent current flow through said switch means until said potential exceeds a minimum value.
 13. Apparatus as in claim 12, wherein said diode means comprises a zener diode.
 14. Apparatus as in claim 12, wherein said diode means comprises two back-to-back parallel connected diodes.
 15. An AC power circuit comprising electrical contacts adapted to be connected to a load and an AC power supply, an electrical device connected in series with said load and said supply, contactless switch means connected to form a current path which bypasses said contacts and including bias means responsive to a potential across said contacts and to current flow through said device, whereby said switch means conducts only from the time of opening of said contacts and initiation of an arc and until the next subsequent zero crossing of the alternating current.
 16. Apparatus as in claim 15, and further including diode means connected in series with said switch means.
 17. Apparatus as in claim 16, wherein said device comprises resistance means connected in series with said contacts, said switch means comprising a TRIAC having its gate connected to respond to current flow through said resistance means.
 18. Apparatus as in claim 17, wherein said contacts and said resistance means form an overload switch.
 19. Apparatus as in claim 16, wherein said device comprises a resistor.
 20. Apparatus as in claim 16, wherein said device comprises a current transformer.
 21. Apparatus as in claim 16, wherein said device comprises a potential transformer and a resistor.
 22. Apparatus as in claim 16, wherein said switch means is adapted to be connected directly across said switch contacts.
 23. Apparatus as in claim 16, wherein said switch means is adapted to be connected across said switch contacts and said device.
 24. An AC power circuit for an electric motor, comprising a motor winding, overload switch contacts and a device mounted within said motor, said winding and said contacts and said device being connected in series and being adapted to be connected to an AC power supply, switch means mounted within said motor and connected to form a bypass current path around said contacts, said switch means including bias means responsive to a potential across said contacts and to current flow through said winding and said device, whereby said switch means is biased to conduction by a potential due to the initiation of an arc and continues to conduct until the next zero crossing of the AC wave.
 25. Apparatus as in claim 24, wherein said switch means comprises a TRIAC, and further including diode means connected in series with said TRIAC.
 26. Apparatus as in claim 24, wherein said device comprises a heater for heating said overload switch.
 27. A circuit for preventing damage to switch contacts when opening under heavy current load conditions, comprising an electrical component adapted to be connected in series with said contacts and a load, voltage and current responsive switch means adapted to be connected to form a bypass current path around said contacts, said switch means including main terminals adapted to be connected across said contacts and a control terminal connected across said component, said switch means being biased to conduction only when a potential exists across said contacts due to arcing and when current flows through said component, whereby said current load is transferred from said contacts to said switch means from the initiation of an arc And until the next subsequent zero crossing of the alternating current.
 28. Apparatus as in claim 6, wherein said main terminals are connected across both said contacts and said current transformer.
 29. Apparatus as in claim 6, wherein said main terminals are connected across said contacts only.
 30. In a power circuit including an AC power supply, a load connected to be energized by said supply, and switch contacts connected in series with said supply and said load for controlling the flow of current to said load, the improvement of means for preventing arcing across said contacts during opening under heavy load current conditions, comprising an electrical component connected in series with said contacts and said load, a triac including main terminals and a gate, said main terminals being connected in said power circuit to form a current path around said contacts, and said gate being connected to said component, whereby said triac is turned on when opening of said contacts produces a potential across said main terminals and current flow through said component provides gate triggering current.
 31. In a power circuit as in claim 30, wherein said main terminals are connected directly across said contacts, said component is a resistor, and said gate triggering current is in phase with the voltage across said resistor.
 32. In a power circuit as in claim 30, wherein said main terminals are connected across both said contacts and said component, whereby current flow through said triac bypasses both said contacts and said component. 